1. Field of the Invention
The present invention relates to a switching element array and a liquid crystal display using the same.
2. Description of the Related Art
A liquid crystal display, an EL display, etc. are known in the art in which pixel electrodes are selected and driven so as to form patterns on a display screen. As the driving method of pixel electrodes, there is known an active matrix driving method in which independent pixel electrodes are arranged in a matrix, and each provided with a switching element to drive the pixel electrode selectively. In such a liquid crystal display, by applying a voltage between the pixel electrode selected by the switching element and a counter electrode, display medium such as liquid crystal sealed between the electrodes is optically modulated. This optical modulation is visually observed as a display pattern on the screen.
Such a driving method enables the display to have a good contrast in its picture, and finds many applications such as in television, etc. As the switching elements for selecting and driving the pixel electrodes, three-terminal type of TFTs (thin film transistor), MOS transistors, etc. and two-terminal type of MIMs (metal-insulating layer-metal), diodes, varistors, etc. are generally used.
FIG. 13a is a perspective view showing an exemplary liquid crystal display employing the active matrix driving method. FIG. 13b is a cross-sectional view of the display of FIG. 13a. As is shown in FIGS. 13a and 13b, a transparent insulating substrate 21 and a counter substrate 26 face each other, and a liquid crystal layer 28 is sealed therebetween. On the transparent insulating substrate 21, a plurality of source electrodes 22 and a plurality of gate electrodes 23 are arranged so as to cross each other at right angles. At each rectangular region bounded by adjoining source electrodes 22 and gate electrodes 23, a pixel electrode 24 for driving liquid crystals is provided. Each pixel electrode 24 is provided with a TFT 25 as a switching element. An orientation film 30 is formed on the region provided with the pixel electrode 24 so as to fully cover the region.
A counter electrode 27 is provided on the counter substrate 26 so as to face the pixel electrode 24 on the insulating substrate 21. An orientation film 31 is formed on the counter electrode 27.
A seal resin 29 is provided at peripheral portions of the liquid crystal layer 28 sandwiched between the counter substrate 26 and the insulating substrate 21, thereby sealing the liquid crystal layer 28.
Such a liquid crystal display is operated as follows: First, a single gate electrode 23 is selected, a gate signal is applied to the selected gate electrode 23, and all of the TFTs 25 connected to the gate electrode 23 are turned to an ON-state. By applying a source signal synchronized with the gate signal via a source electrode 22, each pixel electrode 24 provided with the TFT 25 receives the source signal. As a result, a potential difference required for the display is gained between the pixel electrode 24 and the counter electrode 27. The corresponding charge is stored in the liquid crystal capacitance of the liquid crystal layers 28 provided between the electrodes 24 and 27, and thus the display signal is written in a pixel corresponding to the liquid crystal capacitance. Even when the TFT 25 is turned to an OFF-state, a voltage applied during the ON-state of the TFT 25 is maintained by the charge in the liquid crystal capacitance.
Thus, pixels are driven over a field. Thereafter, in the same manner as above mentioned, the gate electrodes 23 are scanned successively and a gate signal is applied to each of the selected gate electrodes 23; source signals synchronized with the gate signals are applied to the source electrodes respectively; and consequently an image is formed on the display.
Next, an exemplary structure for the TFT is described referring to FIG. 14. FIG. 14 shows a cross-sectional structure of a portion of a conventional active matrix substrate, where the TFT is formed. A gate electrode 51 is formed on a glass substrate 50 as a transparent insulating substrate (corresponding to the insulating substrate 21 of FIGS. 13a and 13b). The gate electrode 51 is branched from gate electrodes arranged on the glass substrate 50. The gate electrodes and source electrodes (not shown) are arranged so as to cross each other. Each of a plurality of pixel electrodes 60 is provided at a rectangular region bounded by the gate and source electrodes, and thus the pixel electrodes 60 are arranged in a matrix. The gate electrode 51 is formed by patterning at the same time as when the gate electrodes are formed. Finally, the TFT provided with a source electrode 58 and a drain electrode 59 is formed on the gate electrode 51 via a gate insulating film 54.
Next will be described an exemplary structure and production method for the TFT in detail referring to FIG. 14. First, the gate insulating film 54 is formed on the gate electrode 51. Then, a semiconductor layer 52 is formed by patterning on the gate electrode 51 via the gate insulating film 54. Subsequently, an etching stopper layer 56 is formed on the semiconductor layer 52, and then contact layers 53a and 53b are formed by patterning so as to be adjacent to either side of the etching stopper layer 56. Finally, a source electrode 58 and a drain electrode 59 are respectively formed by patterning on the contact layers 53a and 53b, and thus the TFT is formed.
Next, an insulating protective film 55 (layer insulating film) is formed on the entire surface of the glass substrate 50 and covers the TFT. The pixel electrode 60 is formed by patterning a transparent conductive film such as an ITO (Indium Tin Oxide) film on the insulating protective film 55. The pixel electrode 60 is electrically connected to the drain electrode 59 through a contact hole 57 formed in the insulating protective film 55.
In a liquid crystal display using such a TFT, patterning of the semiconductor layer 52, the gate electrode 51, etc. requires photolithographic steps. The formation of the above-mentioned TFT requires 6 photolithographic steps including one step for forming the contact hole 57. Generally, a liquid crystal device provided with such a switching element requires 5 to 7 photolithographic steps, which makes the structure and the production process thereof complicated. Therefore, it is difficult to increase the production yield of the device, which leads to an expensive production cost.
As a switching element for the liquid crystal device, an MIM (metal-insulating layer-metal) element may be employed, which has a simpler structure than that of the TFT element and can be produced at low cost. However, the display employing the MIM element as the switching element thereof is inferior in display image quality to the device employing the TFT element due to the asymmetry of current-voltage properties. Recently, there has been a demand for larger-sized liquid crystal displays. However, it seems difficult to obtain larger-sized liquid crystal displays having high picture quality and high production yield at low cost with use of the conventional switching elements such as the TFT or MIM elements.
In order to produce large-sized liquid crystal displays at low cost, switching elements should be produced with high production yield at low cost. Thus, it is desirable to provide a switching element having a simple structure and production process.
It is therefore an object of the present invention to provide a switching element requiring less photolithographic steps than the prior art, thereby providing a liquid crystal display at a low unit cost.